Perceptual Coherence : Hearing and Seeing

288 Perceptual Coherence
One way to strip away the surrounding components and make them
form a separate auditory source is to manipulate the temporal properties of
the components. The most powerful acoustic cue for the fusion of component
sounds is onset asynchrony (see chapter 9). In nearly all instances, if
frequency components start at the same instant, they are perceived as coming
from the same object. On this basis, we would expect that if the target
component started before or after the remaining components, it would
be treated as a separate entity, and performance would then mimic that of a
single component presented alone. D. M. Green and Dai (1992) found that
leads and lags as small as 10 ms disrupted detection (this is slightly shorter
than the 20–30 ms delay that causes components to split apart in other
tasks discussed in chapter 9). Hill and Bailey (2002) attempted to create a
separate stream by presenting the target component to one ear and the
flanking components to the other ear (termed dichotic presentation). If the
target and flanking components were synchronous, dichotic presentation
was only slightly worse. If the components were presented asynchronously,
detection was much worse, but there was no added decrease due to dichotic
presentation. Onset asynchrony between two sounds is a much stronger
basis for the formation of two sounds than is different spatial locations
(also discussed in chapter 9).
A second way to strip away the surrounding components is by means of
coherent amplitude modulation. There are several possibilities: (a) the target
component is modulated, but the nontarget components are not (or the
reverse); or (b) all the components are modulated, but the target component
is modulated out of phase to the other components. For all of these conditions,
the detection of the increment in intensity is impaired.
Comodulation Masking Release
The fundamental lesson from profile analysis research is that the acoustic
signal is typically treated as representing a single source and that listeners attend
to the entire spectrum. Isolating individual components leads to degradation
in the ability to detect the amplitude change of those components.
We can also demonstrate that the acoustic signal with a coherent temporal
pattern is treated as a single source from the reverse direction. Suppose
we have a target masked by noise. How can we make that target more discriminable?
If adding more tonal components to form a coherent spectral
shape makes one tonal target more discriminable, then, paradoxically,
adding noise that combines with the masking noise to form a coherent
noise source also should make the target more detectable. The trick is to
amplitude modulate both the original masking noise and the added noise,
identically in frequency, phase, and depth, to make the coherent noise into
a source. The coherent noise source now seems separate from the target.